Excimer lasers are pulsed gas lasers that deliver radiation in the ultraviolet (UV) region of the electromagnetic spectrum. There are applications of such lasers, for example, laser annealing, that would benefit from a pulse-energy greater than available from the highest energy excimer laser presently available. A greater pulse-energy can be supplied by combining the output of two or more excimer lasers. The output of the lasers must be overlapped spatial and temporally. The spatial overlap can be achieved precisely using an arrangement of optical elements, and optical overlap methods are well known in the art.
The temporal overlap is less precise and depends on the precision which operating characteristics of pulsed power supplies, among other factors, can be reproduced from pulse to pulse. Variation of such characteristics leads to temporal variation of the precision of pulse-overlap. This temporal variation is usually referred to by practitioners of the art as jitter. Depending on the pulse duration and on application requirements, there is usually a jitter value that can not be exceeded without compromising the application.
By way of example, in a laser annealing application, it may be required to combine the pulsed output of two excimer lasers having a pulse repetition rate of about 600 Hertz (Hz) and a pulse-duration of about 30 nanosecond (ns) full-width at half-maximum (FWHM), with pulses having a complex temporal pulse-shape. Maintenance of a complex temporal pulse-shape is possible only if the pulses are overlapped precisely in time. The tolerance of an application toward variations in the temporal pulse-shape in the overlapped beam cannot be stated in general, and depends on the sensitivity of the application with respect to the pulse shape. For applications in the laser annealing area, this tolerance level corresponds to jitter of a few nanoseconds, for example less than about 6 ns for the 30 ns pulse-duration.
In order to understand the factors affecting jitter, it is useful to consider characteristics of pulsing circuit arrangements for an excimer laser. A description of one such circuit arrangement is set forth below with reference to FIG. 1, which is a circuit diagram depicting one typical arrangement 10 of circuitry for delivering high voltage pulses to discharge electrodes of an excimer laser.
Here, high-voltage power (HV-IN) power from a high-voltage power supply (HVPS) is supplied to a terminal 12. The power is used to fully charge a storage capacitor C0 to a predetermined voltage, typically between about 1500 and 2300 Volts (V). The capacitor is charged via a magnetic isolator 14. Magnetic isolator 14 includes a diode D2 and a transformer L6, one side which is connected to a switch 16 including an isolated-gate bi-polar transistor (IGBT) and rectifier bridge, and other components (not shown). An inhibit signal can open or close switch 16 as required.
Magnetic isolator 16 switches the impedance value between the HVPS and storage capacitor by a factor of about 50 from a low value to a high value (and vice versa) depending on whether switch 16 is respectively closed or open. The low impedance value is needed to charge capacitor C0 with high precision. The higher impedance value is necessary to protect the power supply from energy reflected back from the laser discharge, which could otherwise cause very high and potentially destructive peak currents through the power supply. Only sufficient description of magnetic isolator 14 is provided here to understand the operation of circuitry 10. A detailed description of the magnetic isolation principle is provided in U.S. Pat. No. 6,020,723, assigned to the assignee of the present invention, and the complete disclosure of which is hereby incorporated herein by reference.
When charging of capacitor C0 is complete, magnetic isolator 14 is switched to the high impedance condition. Discharging of capacitor C0 is controlled by an IGBT and diode (“free-wheeling diode”—FWD) module IGBT-1. On receipt of a pulse-trigger voltage at the gate of IGBT-1 the IGBT is closed and capacitor C0 is discharged through a magnetic-assist L5, a pulse-transformer L4, and a diode D1. The resultant pulse from transformer L4 is sent to a pulse-compressor 18. The pulse is compressed in three stages formed by saturable inductor or magnetic switch L1 and capacitor C1, saturable inductor L2 and capacitor C2, and saturable inductor L3 and capacitor C3. The compressed pulse is delivered from pulse compressor 18 to the excimer laser tube which includes the laser discharge electrodes and other electrically reactive components.
A reset signal is applied to terminal 20 from a DC power supply (not shown). The signal causes a current of about 10 amps (A) to flow through L5, L1, L2, and L3. This current effectively drives the magnetic cores of these devices from a position in the B-H (hysteresis) loop thereof following a pulse compression, back into the opposite corner of the B-H loop. A sufficient reset is a precondition for obtaining reproducible transition times between one pulse and the next through the pulse-compressor and minimizing jitter between pulses
Considering now problems that would be encountered in trying to drive two excimer lasers, each with a circuit arrangement similar to that of arrangement 10 of FIG. 1, one prerequisite for a low jitter time between pulses delivered by the pulse compressor 18 of each is that the charged voltage of storage capacitor C0 of each must be as reproducible as possible from pulse to pulse. These storage capacitors are each charged in approximately 1 millisecond (ms) via the HSVP, which may be regarded as a regulated current source. The achievable control precision (voltage regulation accuracy) of the high voltage of typical such HSVPs is about ±0.1% of the maximum high-voltage value of the power supply, which is usually about unit of 2.3 kilovolts (kV). A typical controlled value is about 1.6 kV, i.e., well below the maximum.
The voltage-time area (the magnetic saturation flux) of transformer cores of stages of pulse compressors 18 is essentially a constant, with only a small drift or variation resulting from a change in temperature of the core material. These variations are sufficiently slow to enable relatively easy correction for example by controlling the relationship of independent discharge-trigger signals of the circuits. The saturation flux (Ψ) of a compressor stage can be represented by the following equation:Ψ=N∫2BsdA=∫Udt=const.  (1)wherein N represents the number of turns of the respective compressor stage saturable inductor; Bs is the saturation-induction of the core material used; and A is the magnetically-effective core cross-section.
The required time for saturation of the core, and hence transition to a low-inductive state, is obtained from the integral of Udt (the voltage-time area), wherein U is the voltage over this saturable inductivity, which, in turn, is proportional to the charging voltage at the capacitor C0. It follows from this that a variation of this voltage leads directly to a change in the saturation time (the time for the through-connection of the inductances L5, L1, L2, and L3), with the change in time being about 1/U. In the example under consideration, a voltage change of as small as 1 V at capacitor C0 would lead to a time change of between about 5 and 7 ns for the pulse passage through the entire pulse compressor 18.
Because each of the two lasers to be synchronized has an independent high-voltage power supply unit, relative voltage fluctuations in a range of 4.6 V can occur. This would lead to up to 32-ns fluctuations in the time difference between the light pulses delivered by each of the lasers. This is a random pulse-to-pulse phenomenon and can not be predicted, and accordingly can not be corrected.
In theory at least, the above described jitter problem could be rectified by using HVPSs which would allow a pulse to pulse voltage fluctuation (at each C0) of less than about 0.015%. Realization of such power supplies, however, is technologically achievable only with great difficulty and at significant expense. This is because of required high charging power of approximately 50 kilowatts (kW) and charging voltages on the order of 2 kV. There is a need for a solution to the excimer laser synchronization problem that does not require the development of improved power supplies or any other laser components.